Industrial tanks must often be cleaned, commonly after the tank is empty. To do so the interior of the tank or other container is usually sprayed with high pressure cleaning liquids to dislodge the contaminants, both liquid and solid, from the tank surfaces. Because the impact of the high pressure spray is diffused if there is a liquid layer within the tank, it is desirable to keep the amount of cleaning liquid which gathers in the bottom of the tank to a minimum.
Removing cleaning liquid from the bottom of the tank is often accomplished by using positive displacement pumps. However, positive displacement pumps are often sensitive to solid contaminants in the liquid being pumped because of the small clearances necessary in their construction. Further, the valves commonly used are also sensitive to foreign solid material. These foreign solids can often lead to the early destruction of the pump. Centrifugal pumps are sometimes used; however, they generally do not work well when air is mixed with the liquid. Therefore, what has been missing in the prior art is a cleaning system insensitive to both drawing solid contaminants from the bottom of the tank and to drawing air in with the cleaning liquid.
Oil tankers pose special problems for tank cleaning apparatus. Oil as pumped out of the ground commonly contains rocks, sand and gravel. In addition to this naturally present foreign matter, a great number of rags find their way into the oil holding compartments of oil tankers. The presence of contaminants of types which both float and sink create additional problems during cleaning operations.
Railroad tank cars are typically cleaned by transporting them to special cleaning facilities. Just getting the tank car to the cleaning facility is expensive and time-consuming. However, highly mobile tank car cleaning units, which could be driven to the railroad tank car, are not generally available.